1. Field of the Invention
The present invention relates to wireless telecommunications and, more particularly, to methods and systems for selective call routing for transcoder free operation.
2. Description of Related Art
Mobile stations often transmit and receive voice and other media in a compressed digital format in order to reduce bandwidth. In particular, a mobile station may receive voice or other media from the user, digitize it, compress the digital signals in accordance with various algorithms, and then transmit the voice or other media in the compressed digital format. A mobile station that receives media in the compressed digital format decodes it in order to obtain the original signal. Mobile stations typically include vocoders to convert the voice or other media between analog and compressed digital formats. A number of different compressed digital formats are commonly used. One example is the Enhanced Variable Rate Codec (EVRC) format, which supports voice transmission at a data rate of about 8 kilobits per second (Kbps). The EVRC format is described in the standard “Enhanced Variable Rated Codec, Speech Service Option 3 for Wideband Spread Spectrum Digital Systems,” TIA/EIA/IS-127 (published January 1997), which is incorporated herein by reference.
In contrast, circuit-switched telephone networks, such as the public switched telephone network (PSTN), typically carry voice or media in an uncompressed digital format, such as a pulse code modulation (PCM) format. For example, in a typical PCM format, a voice signal is sampled at a rate of 8000 times a second and digitized with an 8-bit resolution, resulting in a 64 Kbps signal. Trunks used in the PSTN are often able to carry many of these 64 Kbps signals at a time, in separate channels. For example, a T-1 trunk is able to carry 24 of these 64 Kbps channels.
Because of the differences between these digital formats, wireless telecommunications systems often use transcoders to convert between the compressed digital formats used by mobile stations and the PCM format used by circuit-switched telephone networks. Using transcoders can result in inefficiencies, however. For example, if a call involves two mobile stations, then the voice or other media exchanged in the call may be transcoded twice. One transcoder may convert the media from the transmitting mobile station to PCM format for transmission through the PSTN, and another transcoder may convert the media from the PCM format to the compressed media format for transmission to the receiving mobile station. Using transcoders “in tandem” in this way is often undesirable because each transcoder can add delay to the transmission and can degrade the quality of the voice or other media.
To address these concerns, various proposals have been made to provide “transcoder free operation” (TrFO) in calls between mobile stations. In a transcoder-free call, no transcoders are present in the communication path. Thus, the two mobile stations exchange media during the call using a common compressed digital format, without transcoding. One such approach is described in 3rd Generation Partnership Project, “Out of band transcoder control,” 3GPP TS 23.153 v5.6.0 (September 2003), which is incorporated herein by reference. In this approach, the media, in the common compressed digit format, is transported through a packet-switched network between media gateways that are controlled by media gateway controllers. Before a call between two mobile stations is established, the media gateway controllers use out-of-band signaling to select a common compressed digital format, i.e., to negotiate a codec that both mobile stations will use for the call. During the call, the mobile stations exchange media, using the common compressed digital format, via the media gateways and the packet-switched network.
One difficulty with this TrFO approach, however, is that it requires “next generation” network elements, such as media gateways and media gateway controllers that may not be present in “legacy,” e.g., IS-41 based, wireless networks. Many wireless service providers are in the process of upgrading their networks to include such next generation network elements to allow for TrFO functionality. However, this upgrade process is likely to occur in a piecemeal fashion over the course of many years, with different service areas being upgraded at different times. As a result, “next generation” and “legacy” network elements are likely to co-exist for quite some time. This means that a mobile station could be served by either a legacy MSC or a next-generation MSC, i.e., one with access to a media gateway, depending on the wireless service provider and/or on where the mobile station is operating. Accordingly, there is a need to provide transcoder free operation in wireless networks that include both legacy and next-generation network elements.